How can I get the Exchange in SBS to be less strict in filtering spam?
Most clients emails go directly into the spam folder.
It depends on the Exchange version you are using. SBS 2011 uses Exchange 2010, SBS 2008 uses Exchange 2007 and SBS 2003 uses Exchange 2003.
Setting the "SCL Junk Threshold" parameter for one of the Junk/Quarantine/Reject/Delete actions will influence your filter's sensibility either organization-wide
Set-OrganizationConfig -SCL[Junk|Quarantine|Reject|Delete]Threshold <Value 1-10>
Set-Mailbox -Identity John -AntispamBypassEnabled $true -SCL[Junk|Quarantine|Reject|Delete]Enabled $true -SCLJunkThreshold <Value 1-10>
The SCL is the "Spam Confidence Level" calculated by Exchange anti-spam modules - the higher you set the threshold, the less sensitive your filter will be. Note that you have rather limited possibilities to influence the SCL calculation itself. See the Technet docs for details.
pretty much the same thing as Exchange 2010, but without the ability for user-specific configurations. The organization-wide setting can be done with
Set-ContentFilterConfig -SCL[Reject|Delete|Quarantine]Enabled $true -SCLQuarantineThreshold <Value 1-10>
Technet docs for details again.
Exchange 2003 SP2
Basically, only the "Block threshold" is available and configurable either through the Exchange System Manager or the AD ms-Exch-Uce-Block-Threshold attribute.